Mass spectrometry



Jan. 24, 1961 vw. A. MORGAN MAss sPEcTRoMETRY Fled- Sept. 5, 1958 ATTORNEY.

MASS SPECTROMETRY Walter Avery Morgan, Baytown, Tex., assignor, by mesne assignments, to Esso Research and Engineering Company, Elizabeth, NJ., a corporation of Delaware Filed Sept. 5, 1958, Ser. No. 759,245

1 Claim. (Cl. Z50-41.9)

This invention relates to mass spectrometers. More particularly, this invention relates to an improved mass indicator for use with mass spectrometers.

One method of determining the chemical or elemental composition of a material as Well as the relative abundances of the various materials in the sample is by means of a mass spectrometer. In a mass spectrometer the material to be analyzed is ionized, accelerated by an electrostatic field, and resolved into homogeneous ion beams by means of a magnetic field. Each homogeneous beam contains ions having a single mass to charge ratio. By varying the strength of the electrostatic field used to accelerate the ionized particles, ion beams of various mass to charge ratio can be brought to focus on the ion collector and their relative intensities determined.

One major objective of mass spectrometry is to provide a visual form of mass to charge ratio identication at all times. The visual form should indicate directly the particular mass to charge ratio of the ionized particles brought to focus on the ion collector.

This invention provides the art of mass spectrometry with a new and relatively simple system for indicating visually the mass to charge ratio of the ionized particles being focused upon the collector at any particular time.

'Ihe mass indicator system to be described herein is particularly useful with a mass spectrometer including an ionization chamber, an analyzer, and a collector. Means are provided for accelerating and focusing the ionized particles by means of an electrostatic field. A field-applying electrical circuit is provided for applying a field through the analyzer to separate the ionized particles according to their mass to charge ratio.

The new mass indicator includes a means for obtaining -a substantially iXed voltage from the field-applying circuit. Means are also provided for obtaining a voltage from the accelerating electrical circuit. These two voltages are continually compared by a voltage comparator such as la differential amplifier. An electrical circuit, including the voltage comparator, is provided, which is responsive to changes in the voltage from the accelerating electrical circuit to indicate the mass to charge ratio of ionized particles striking the collector at any particular time.

A better understanding of the invention, as Well as its many advantages, may be had by reference to the following detailed description and drawing which is a diagram of a simplied mass spectrometer equipped with a mass indicator of my invention.

Referring to the drawing, a mass spectrometer is shown including an analyzer 10. The material to be analyzed, such as a gaseous hydrocarbon, is admitted into the ionization chamber 12 by means of an inlet, such as gas inlet 13. The gaseous molecules in ionization chamber 12 are ionized in the ionization chamber 12.

An electrostatic tield impressing electrical circuit is provided for accelerating the ionized particles through exit slit 14. This circuit includes a variable D.C. voltage source 15 which feeds a voltage through lines 16 and 17 arent to apply a difference in potential V between the chamber 12 and accelerating electrode 18.

After the ionized particles have accelerated they are resolved into separate homogeneous beams according to their mass to charge ratio by means of a magnetic eld produced between the poles of the electromagnet 19. The strength of the magnetic eld which exists between the poles of the electromagnet 19 is a function of the current tlowing through the winding 20. By maintaining the magnetic eld constant and varying the accelerating potential V the resolved ion beam is caused to move across the resolving slit 21. Those ions which pass through the slit impinge on the collector electrode 22. The current pro'- duced by the ions impinging on the collector 22 is measured by an electrometer amplifier and recording apparatus 23.

A resistor R3 is connected in series with the electromagnet eld windings 20. Current from the magnet power supply constant current regulator 24 passes through the tield windings 20 yand resistor R3 producing a voltage between tap 25 and ground which is proportional to the magnetizing current and which is a function of the magnetic field strength existing between the poles at the electromagnet. Since the magnetizing current is maintained constant by the current regulator 24,- the magnetic iield strength and the voltage from the tap 25 to ground will also be constant. The position of theV tap 25 on resistor R3 is adjusted whenever the magnetizing current is changed to a new value so that the digital dial 26, used to indicate the mass to charge ratio of the ions being brought to focus, will always read mass to charge ratio direct-ly. Once adjusted for a particular magnetizing current the position of the tap 25 is fixed and the voltage between tap 25 and ground is a constant value.

A pair of resistors R1 and R2 are connected across the accelerating voltage V. R2 is a linear potentiometer with movable wiper arm 27. The voltages from wiper arm 27 and tap 25 are fed to a voltage comparator 28. The output from the voltage comparator 2S is connected to a servo motor 2.9 which in turn is connected to wiper arm 27 and the digital dial 26.

The voltage comparator 28 may suitably be a differential amplifier. Any slight difference in voltage between Wiper arm 27 and ground and tap 25 and ground is amplified in the voltage comparator and causes the servo motor 29 to rotate the wiper arm 27 in such a direction that the magnitude of the diiferential voltage at the input to the comparator is reduced to zero. The digital dial 26, attached to the potentiometer shaft, measures the mechanical position of the potentiometer wiper and expresses that position in terms of mass to charge ratio of the ions being brought to focus on the ion collector 22.

That the mass is directly and visually indicated can be shown mathematically. In the mass indicating circuit shown,

By the use of the comparator circuit 28 and servo motor 29, the voltage Ey is kept substantially at zero. Therefore,

Patented Jan. 24, 1961 Fromastndy. ofaionopticsitrcanbe .shownihat kK Vfn where Ki-a'constant, and

:the masslto.. charge' ratio of" the particles being brought to focus atjth'e ion collector" Bv=1the magnetic ld in gauss; and 4 r=the radius of the analyzerin centimeters Fori any yparticular application, B, r, ER; Rband Rz, are constant... Therefore,v Ris proportional .to M. Thus, the. position of. wiper; armr2'7 on resistor. R2A which. is controlled. by. the-.servo-motor 29 is a-direct indication of the mass to charge ratioof the ionized particles being detectedratany particular time. This mass indicated bythe digital dial 26.v

In; operation, Yif itis desired to scanthe mass spectrum of. the,.sample,1 vo1tage.V- varied. Asa the. voltage changes, `thewiper. arm .27 isscontinuously moved by; the servofmotorf-Zgto maintain the-voltage Eff-0a The move.- ment-of. Wiperdarm 27- is..calibrated-.`.to;-ndicate on .the

4 digital dial 26 the mass to charge ratio of the ions being brought to focus at the ion collector 22.

I claim:

A mass indicator system comprising: an analyzer; an ionization chamber; a collector; a voltage source for impressing a potential toaccelerate the ions in the ionization chamber and to direct'theionized particles through the analyzer tothe collector; a magneti juxtaposed on said analyzer; a magnet voltage circuit for applying a magneticield to; the analyzertoseparate-the ionized particles into homogeneous" beams: according. to their mass to charge ratio; a resistor forming part of `themagnet voltageLfcircui-tgat .least oner'resistoriconnected across the voltage source for impressing a potential to accelerate the ions; and a mass indicatorcircuitincluding a tap for obtaining a substantially fixed voltage from the resistor in the magnet voltage circuit, a wiper arm for obtaining a voltage. from thev resistor connected across the voltage source; a comparator fof *receiving both voltages,v a. com

20 parator controlledv motor connectedto thevv wiper arm responsive -Vto `any difference inthe .two voltages .to `move the -Wiper arm until the -two voltages. areagain equal, and a dial indicator connected to the Wiper arm fordirectly indicating the mass to. charge ratio of ionizedY particles 25 striking the collector.

Langmuir June 26, 1945` 2;'676Q264' Berry Apr: 20,1954 

